The 500-hPa circulation pattern during May featured
above-average heights over northeastern Canada, across the Mediterranean
Sea, and over western Russia, and below-average heights across the central
North Pacific, the eastern United States and Scandinavia (Fig. E9).
These circulation anomalies were associated with a strong negative phase
(-1.3) of the North Atlantic Oscillation (NAO) and a strong positive phase
(+1.7) of the Pacific/ North American teleconnection pattern (PNA) (Table
E1, Fig. E7). Another
prominent aspect of the circulation pattern was cyclonic streamfunction
anomalies at 200-hPa extending from the eastern subtropical North Atlantic
to central Asia in the Northern Hemisphere, and from the eastern subtropical
South Atlantic to the central Indian Ocean in the Southern Hemisphere (Fig.
T22). The pronounced inter-hemispheric
symmetry to this pattern suggests a link to the suppressed convection seen
across the central and eastern equatorial Atlantic and equatorial western
Africa (Fig. T25).

The main surface temperature departures during May are
above-average temperatures across Alaska, southern Europe, western Russia,
and both the high latitudes and subtropical latitudes of the North Atlantic
(Fig. E1). The main precipitation anomalies
reflected above-average totals in the northwestern U.S., northern Europe,
and central China, and below-average totals in the eastern U.S. and southern
Europe (Figs. E3, E5, E6).

a. Pacific/ North America

The main 500-hPa circulation anomalies affecting the
Pacific/ North American sector during May included above-average heights
across Alaska and eastern Canada, and over the southwestern U.S., and
below-average heights across the central North Pacific and the northeastern
United States (Fig. E9). In the middle
latitudes, this anomaly pattern reflected a strong positive phase (+1.7) of
the Pacific/ North American teleconnection pattern (PNA) (Table E1,
Fig. E7). This pattern was associated with an
enhanced East Asian jet stream across the central and eastern North Pacific,
and with an eastward extension of the jet exit region to the eastern North
Pacific (Figs. E10, T21).

This circulation resulted in above-average precipitation
throughout the northwestern U.S., with monthly totals in the Pacific
Northwest exceeding the 90th percentile of occurrences
(Fig. E3). Area-averaged monthly totals in
the Inter-Mountain region of the western U.S. reached the 80th
percentile of occurrences, making this the 7th
consecutive month of above-average precipitation in that region (Fig. E4).
Southern California also experienced a continuation of above-average
precipitation during May. This region has also recorded large precipitation
surpluses since October 2004. In contrast, monthly precipitation totals were
generally below average over the eastern United States in the region
upstream of the mean upper-level trough axis.

Both Alaska and northeastern Canada experienced well above-average
temperatures during May, with departures in both regions averaging 2°-3°C
above normal and exceeding the 90th percentile of
occurrences (Fig. E1). In contrast,
exceptionally cool temperatures were observed across the eastern U.S. in
association with anomalous northwesterly flow upstream of the mean
upper-level trough axis.

b. North Atlantic, Eurasia

The 500-hPa circulation pattern during May featured a
strong negative phase (-1.3) of the NAO, with above-average heights over the
high latitudes of the western and central North Atlantic, and below-average
heights extending across the eastern U.S. and western North Atlantic (Fig.
E9). Farther downstream, persistent negative
height anomalies were centered over Scandinavia and persistent positive
height anomalies were observed across southern Europe and most of western
Russia. These conditions contributed to above-average precipitation in
northern Europe and below-average precipitation in southern Europe. They
also contributed to exceptionally warm temperatures throughout western
Russia, with monthly values averaging +2°-3C above average and exceeding
the 90th percentile of occurrences.

Exceptionally warm SSTs were again observed throughout the high latitudes
and subtropical latitudes of the North Atlantic, with departures in both
regions exceeding the 90th percentile (Figs. T18,
E1). This anomaly pattern reflects the ongoing
warm phase of the Atlantic multi-decadal mode that began approximately in
1995 (Goldenberg et al. Science, 2001).

2. Southern Hemisphere

The mean 500-hPa circulation pattern during May featured
above-average heights from Australia eastward to the central South Pacific,
and over the central South Atlantic, and below-average heights over the low
latitudes of the South Pacific, southern South America, and the central
Indian Ocean (Fig. E15). The pattern of
positive height anomalies was very persistent during the month (Fig. E17),
and was associated with large temperature and rainfall departures across
both Australia and southeastern South America.

In Australia anomalously warm and wet conditions were observed west of
the mean ridge axis and anomalously cool and dry conditions were observed
east of the ridge axis (Figs. E1, E3).
In southeastern South America, anomalously warm and wet conditions were
observed in the area upstream of the anomalous upper-level ridge, with
monthly temperature and precipitation departures both exceeding the 90th
percentile of occurrences in this region. Southern South America also
recorded above-average precipitation during May, in association with
enhanced storminess immediately downstream of the mean upper-level trough
axis.